1. Field of the Invention
The present invention relates to a carrier, a two-component developer using the same, and an image-forming apparatus using said developer. The carrier according to the present invention can be used as a component in a two-component developer preferably suitable for electrophotographic image-forming apparatuses, such as copiers, printers, facsimile machines and the like.
2. Description of the Related Art
In an electrophotographic image-forming apparatus, generally, an image is formed by the steps of charging, light-exposure, development, transfer, cleaning, discharge and fixing. Specifically, for example, the surface of a rotating photoconductor is uniformly charged by a charger, and then exposed to laser light emitted from a light exposure device according to the image information, thereby forming an electrostatic latent image thereon. The latent mage is developed by a developing device into a toner image, which is then transferred by a transfer device onto a recording material, where the toner image is heated to be fixed by a fixing device. The residual toner on the surface of the photoconductor is removed off and collected in a collection chamber by a cleaning device. The cleaned surface of the photoconductor is discharged by a discharger so as to be ready for the next round of the image-forming process.
As a developer for a latent image on a photoconductor, generally a single-component developer comprising toner alone, or a two-component developer comprising toner and carrier are used.
Since single-component developers do not need to be mixed before use, they have an advantage that developing devices used therewith have a simple structure with no mixer or the like. However, they have a problem of being difficult to charge toner stably, etc.
On the other hand, since two-component developers need to be stirred before use in order to homogeneously mix toner and carrier, they have a problem that developing devices used therewith have a complicated structure with a mixer or the like. However, the two-component developers have good charge stability and good applicability to high-speed machines, and therefore are commonly used in high-speed image-forming apparatuses and multicolor image-forming apparatuses.
As a carrier used in a two-component developer, a magnetic particle of ferrite or the like having a particle size of 20 to 100 μm is generally used. The magnetic particle has, on its surface, a coating layer of acrylic resin, silicone resin or the like, so as to reduce the moisture-dependent changes in the characteristics and fusion of toner to the surface. In particular, a carrier composed of a magnetic particle (core particle) coated with a thermoset silicone resin has advantages that such a carrier has excellent durability and a toner component or the like is difficult to adhere on the carriers surface (see, for example, Japanese Patent Laid-Open Publication No. Hei 9 (1997)-6054).
However, when used in a two-component developer, a carrier coated with a thermoset silicone resin markedly increases in the amount of electrostatic charge after approximately 1,000 to 5,000 rounds of the developing process. As the result, the image density decreases. Even if a charge control agent of the same polarity as the toner combined with the carrier is added to the silicone resin in order to suppress the increase in the amount of charge, it is not possible to completely prevent the decrease in image density, since the amount of charge varies among the production lots.